The invention relates generally to the field of digital data processing systems, and more particularly to data storage devices for use in such systems.
In digital data processing systems, that is, computer systems, digital data is stored in a data storage device, such as a memory, which typically comprises a large number of storage locations, each of which is identified by an address. Digital data comprises a plurality of binary data digits, known as bits, that are organized into addressable units of eight-bit bytes, sixteen-bit words, and so forth, with each unit being stored in an addressable storage location. In prior computer systems, the data was stored magnetically, with each data bit being stored in a core, that is, a toroid of magnetic material. The value of the data bit stored in the core was represented by the direction of the magnetization in the core, that is, the direction of the magnetic flux maintained in the core. While core memories were typically relatively bulky, they did maintain their data when power was turned off or during power failures.
In a typical magnetic data storage system, the magnetic cores comprising the system were addressed using a coincident current arrangement. In such an arrangement, the cores were arranged in array of rows and columns, with wires extending through the cores down each column and across each row. To write data into, or read data from a particular core, the wires associated with the core's row and column were energized by applying an electric current to them. The amount of current in both wires was sufficient to alter the magnetization of a previously-magnetized core, and so the magnetization core in the array location associated with the intersection of the wires was adjusted by the currents. However, the current in each energized wire was insufficient to alter the magnetization of a previously magnetized core, and so the magnetizations of the other cores in the row and column associated with the wires are unchanged. The arrangement of the cores in the array effectively operated as an addressing mechanism, since a single bit in an array could be selected by appropriate selection of a wire associated with the core's column and a wire associated with the core's row.
More recently, data storage devices have incorporated, instead of magnetic cores, electronic storage devices which store data bits having data values represented by the value of the electrical charge on a capacitor. This facilitated fabrication of memories using integrated circuits, but the data stored in such devices is erased when power is turned off or during power failure.